iwlwifi: mvm: cleanup roc te on restart cleanup

iwl_mvm_restart_cleanup() calls ieee80211_remain_on_channel_expired()
on cleanup, but it doesn't clean the actual roc time
events, resulting in failure of further ROC attempts.

Refactor iwl_mvm_stop_roc() a bit, and add a new function
to only cleanup the roc time events (without sending further
commands).

Signed-off-by: Eliad Peller <eliadx.peller@intel.com>
Signed-off-by: Emmanuel Grumbach <emmanuel.grumbach@intel.com>